He has swimming lessons on Mondays with his good mate. I just thought it would be fun and a bit funny (he's just a baby...), and that we'd just go and check it out, but it turns out I really see the value in all of the activities (as well as enjoying the outing with our lovely friends), so I'm afraid I've decided that this is a weekly activity I'm happy to keep paying for. Afterwards, we play and picnic together.
My friend and I take turns hanging out with the lads while the other goes for a baby-free swim (when we can be bothered). Feels great!